This invention relates to an apparatus for assisting the natural action of a defective heart and more particularly to a heart pump, such as an intra-aortic balloon which is synchronized with the patient's heart.
An intra-aortic balloon is a device which is designed to augment diastolic pressure and flow in a manner which displaces a given volume of blood from the thoracic aorta in diastole. In addition the rapid collapse of the intra-aortic balloon prior to the onset of systole produces a reduction in systolic pressure and if properly timed, has a vacuum-like effect, assisting in the ejection of left ventricular contents during early systole. In this manner, there is a sharp reduction in the left ventricular systolic afterload, improving left ventricular emptying and thus reducing left ventricular end-diastolic volume over a period of time. With reduction in left ventricular end-diastolic volume and therefore, end-diastolic pressure, and a diastolic augmentation of coronary artery pressure, the subendocardial coronary circulation may be markedly improved.
To achieve these goals the balloon should ideally possess the properties of rapid inflation and deflation. This provides a source of momentum in diastole propelling the blood forcefully into the systemic circulation, and a source of vacuum momentum during deflation to augment the ejection of blood from the left ventricle in systole.
Another property vital to efficient balloon performance is timing of inflation and deflation to correspond exactly with the events of systole and diastole as initiated by the patient's cardiac rhythm. Early inflation of the balloon occuring during the latter part of the ventricular systole, will reduce left ventricular emptying. In a similar manner late deflation of the balloon (that is, during the initial phase of the ventricular systole) will augment rather than reduce ventricular afterload. Late inflation of the balloon after the onset of diastole, may not achieve the same degree of diastolic augmentation. Early deflation of the balloon prior to the onset of systole will produce a premature intra-aortic vacuum which may induce a reversal of coronary flow by allowing the reaccumulation of systemic blood in the aorta will thwart the effectiveness of the systolic afterload reduction.
At present, the methods of controlling in-series cardiac assist devices, particularly of the intra-aortic type, are subjective in nature and adjustments are performed manually by trained operators. Only recently have research efforts been directed toward finding objective criteria for controlling these devices. Clark et al, the "Feasibility of Closed Loop Control of Intra-Aortic Balloon Pumping", IEEE Trans. Bme 20, 404-412 (1973) and Williams et al, the "Control Criteria for Intra-Aortic Balloon Pumping", Proc. 28 Acemb, 349 (1975).
Timing is a critical property in the use of intra-aortic devices. Present commercial devices typically employ two timing adjustments to control the assist device. The first is the time interval from the ECG R-wave to pump activation (termed "delay") and the second is the time interval from activation to deactivation of the device (termed "duration" of the pump systole). A change in either of these adjustments will alter the phase of the driving waveform and hence the afterload phase angle. Patents of interest in this area are U.S. Pat. Nos. 3,426,743; 3,430,634; 3,452,739; and 3,966,358. In these references timing adjustments include the delay from the ECG R-wave to either pump systole or pump diastole.
To perform a well controlled study of the relationship between the afterload phase angle and hemodynamic effects of cardiac assistance a control unit is needed which allows accurate, reproducible and convenient control of the phase angle. Such control is not possible with conventional units. Thus there exists a need for a device capable of controlling the afterload phase angle.